This invention relates to a pile foundation as structural foundation used in a frigid area and, more particularly to a frost damage proofed pile or pile secured against frost damage.
When a rack for a pipeline or a similar structure is installed in a frigid area such as permafrost area or seasonally frozen soil area, it is essential to a protect the structure against frost damage such as freezing, frost heaving or thawing and resulting ground subsidence. To this end, various construction methods have been resorted to, including using a piling foundation, which appears to be most popular.
By permafrost area is meant a terrain or area such as Alaska, Canada or Siberia where the soild layer frozen through the year, hereafter referred to as permafrost, is distributed and where the annually averaged temperature is lower than 0.degree. C. By the active layer is meant the soil layer extending from the ground surface to the permafrost layer. The active layer is affected severely by seasonal changes in temperature and subjected to freezing and frost heaving during winter while being also subjected to thawing and ground subsidence during summer. By the seasonally frozen soil area is meant the terrain where the annually averaged temperature is higher than 0.degree. C. The seasonally frozen soil area is devoid of the permafrost and subjected to freezing during winter and thawing during summer. In the following description, the seasonally frozen soil layer is used synonymously as the active layer.
The pile foundation used in a frigid area is embedded in the permafrost for supporting the weight of the superstructure and the frost heaving force and the negative friction on the basis of the adfreeze strength between the permafrost and the pile surface. To this end, it is essential to provide for a positive freezing strength between the pile and the permafrost and a sufficient length of pile embedment in the permafrost. However, the permafrost is not necessarily of uniform properties, but may have variable values of freezing strength, depending upon the soil property or temperature. Thus the structure may actually be subjected to frost damage even if the pile is embedded into the permafrost a distance designed to give a sufficient adfreeze strength. Thus it is required that a certain safety factor be taken into account when designing the pile length, thus causing disadvantages from the viewpoint of ease of construction and economy. There is a great need for an improved technique and system in setting piles in a frigid area.
U.S Pat. No 3,630,037 discloses a system of protecting a pile from frost heaving during the refreezing of the thawed portion of a permafrost by placing about the pile a rubber sleeve which is slightly longer than the thickness of the portion of the permafrost which melts and refreezes. Also, U.S. Pat. No. 4,585,681 (filed on June 26, 1984, Ser. No. 624,750) discloses a frost damage proofed pile in which the pile surface is covered with a tubular sheath member of a length and a portion of the length is formed as an extensible section which is defined by a series of ridges extending outwardly of the sheath member cross-section with respect to the longitudinal axis of the extensible section.
With the systems disclosed by these prior U.S. patents, a liquid must be sealed into the space between a pile and a rubber sleeve or sheath member and otherwise no effective frost heaving prevention can be obtained. As a result, the sleeve or the sheath member is secured at least only at its lower end to the pile and the greater part of its body portion is not secured to the pile. This requires that a special attention be given for example to the danger of the sleeve or the sheath member leaning or falling off when driving the pile into a bored hole in the ground.